Lead is known to be added in small quantities to copper alloys, such as brass and bronze, in order to favour machining and moulding operations of the pieces made of said alloys. As a consequence of the thermal and mechanical stresses due to the working operations, lead tends to segregate in small globules which emerge to the surface of the pieces. In the case of plumbing components for drinkable water distribution systems, such as taps and valves, this phenomenon is particularly undesirable, because by coming into contact with lead on the surface of said components, water is contaminated in a considerable measure often exceeding the limit of 10 .mu.g/l recommended by the World Health Organization and laid down by the law in force in some countries, such as the NSF61 in force in USA.
The problem of lead contamination of drinking water has been already tackled and a few methods and processes for the treatment of the surfaces of taps and valves aiming to reduce the superficial lead thereof have been proposed.
EP 0 683 245 describes a process wherein the above mentioned components are treated with a water solution of an acid that forms with lead a substantially water-insoluble compound. The preferred acid for such a treatment is an oxy-acid of phosphorus, for example orthophosphoric acid.
In EP 0 695 833 is described a process based on the use of a two-component solution: the first component promotes the superficial lead dissolution and is formed of chloride ions, whereas the second one complexes the ions in solution, subtracting them to the equilibrium of the dissolution reaction and allowing thus more lead to dissolve. The latter function is performed by pyrophosphate ions.
Patent application WO97/06313 describes a process in two steps. In the first step the brass components are treated with a water solution of a non-oxidizing acid, chosen among sulfamic acid, fluoboric acid, methanesulfonic acid, fluosilicic acid, acetic acid and mixtures thereof, or alternatively are treated with a mixture of an oxidizing acid and a peroxide in water, preferably a mixture of citric acid and hydrogen peroxide. In the second step, which has the purpose of passivating the brass component surfaces, a basic solution of sodium hydroxide, sodium silicate or mixtures thereof having a pH within 10 and 13 is used.
Application EP 0 892 084 describes a single-step process, wherein the brass components are treated with the solution of a detergent and an acid, preferably acetic acid or nitric acid or a mixture thereof.
Patent application WO98/30733 describes another process in two steps. The first step consists in the treatment of brass components with a hot solution of sodium or potassium hydroxide, having a pH higher than 10 and suitable for removing dirt and residues from the casting of such components and for removing most of the superficial lead. The second step consists in dipping said components in a water solution of a carboxylic acid containing 1 to 8 carbon atoms and 1 to 4 carboxylic groups, preferably an acetic acid water solution, in order to complete removal of superficial lead.
Among said known processes, some solve the problem of lead contamination of drinking water by pushing the elimination of said polluting metal under the requested limit, but a consequent technical problem is generated, i.e. the alteration of the chemical composition of the alloy resulting in the deterioration of the treated piece. From the tests carried out, it is clear that the treatments made according to the teachings of said prior art patents provide for lead elimination, but at the same time also the other constituents of brass or bronze are attacked. Therefore, owing to these treatments, the alloy chemical composition is modified, resulting in a deterioration of the component aesthetic appearance. In some cases, even a change of the color of said components from yellow to red was noticed, indicating the increased copper percentage. However, none of the above listed patents considers the problem of selectivity in lead elimination.